Bivalent nickel inorganic salts are well-known. They are usually soluble in aqueous medium and have little solubility, if any, in hydrocarbon medium. Conversely, bivalent nickel salts of carboxylic acids of the formula (RCOO).sub.2 Ni, where R is a substituted or unsubstituted hydrocarbyl radical, are usually soluble, sometimes in large amount, in the hydrocarbon media, provided the radical R has a sufficient number of carbon atoms. This property is often desired when the nickel salts must be used as homogeneous catalysts, either alone or associated with Lewis acids, such as alkylaluminum compounds. It is true that the use of insoluble nickel salts as catalysts has several known disadvantages, particularly a weaker activity as compared with soluble salts and difficulties of use, particularly in continuous industrial operations where precise and particularly low proportions of catalyst must be used. This explains why it has already been proposed to prepare soluble oligomerization catalysts, such as catalysts for dimerizing or co-dimerizing mono-olefins, by reacting nickel carboxylates with hydrocarbyl aluminum halides.
The use of these catalysts has however the disadvantage that the activity observed in continuous operations is often lower than in batch operations, and even tends to decrease in the course of time. No satisfactory explanation of this phenomenon has yet been found.